The invention relates to an apparatus for establishing and/or monitoring a predetermined filling level in a container.
The prior art describes apparatus having an oscillating element which is fastened to a diaphragm and projects into the container. One, two or more oscillating bars fastened to the diaphragm can be used, for example, as the oscillating element. The diaphragm is caused to oscillate by means of an electromechanical transducer and the resulting oscillations are picked up and converted into an electrical reception signal. The oscillation of the diaphragm causes the oscillating element to oscillate. If, in operation, the oscillating element is covered by a charge material arranged in the container then its oscillation is attenuated. The oscillation frequency of the apparatus and the oscillation amplitude of the oscillating element change. An electrical reception signal is fed to an electronic evaluation unit and evaluated. A reduction in the oscillation frequency and/or in the oscillation amplitude below a fixed reference value is identified by an electronic evaluation unit connected downstream and is indicated and/or used to initiate a switching operation.
Apparatus of this type are employed in many industries. They are employed, for example, as filling level limit detectors in the chemical industry, in the foodstuffs industry or in the field of water treatment. In these cases, they may serve, for example, as a protection against overfilling or as a safeguard against pumps running dry. The variety of charge materials into which such an apparatus can be immersed during operation ranges from water through yogurt, colorants and coatings to highly viscous charge materials such as honey or greatly foaming charge materials such as beer, for example.
DE-A 44 19 617 describes an apparatus for establishing and/or monitoring a predetermined filling level in a container, which apparatus comprises:
a housing, PA1 two oscillating bars projecting into the container, PA1 a diaphragm which is fixedly clamped into the housing at its edge and PA1 a piezoelectric excitation transducer arranged on the diaphragm and serving to excite the diaphragm to produce oscillations, and PA1 a piezoelectric reception transducer arranged on the diaphragm and serving to receive and convert oscillations thereof into an electrical reception signal, PA1 the oscillations of the diaphragm causing the oscillating bars to oscillate in opposite directions transversely with respect to their longitudinal axis. PA1 a housing, PA1 two oscillating bars projecting into the container, PA1 a diaphragm which is fixedly clamped into the housing at its edge and, PA1 a single disk-type piezoelectric element arranged on the diaphragm and serving to excite the diaphragm to produce oscillations and serving to receive and convert oscillations thereof into an electrical reception signal, PA1 the oscillations of the diaphragm causing the oscillating bars to oscillate in opposite directions transversely with respect to their longitudinal axis. PA1 a housing, PA1 an oscillating element projecting into the container, PA1 a diaphragm, which is fixedly clamped into the housing at its edge and PA1 an electromechanical transducer PA1 in which a percentage interval between a resonant frequency and an antiresonant frequency referring to the resonant frequency exceeds 20% when the oscillating element oscillates in air.
to which the oscillating bars are fastened in a manner spaced apart from one another, PA2 to which the oscillating bars are fastened in a manner spaced apart from one another, PA2 on which element two electrodes of identical form are arranged symmetrically with respect to an axis lying in a diaphragm plane, PA2 to which the oscillating element is fastened, and PA2 for exciting the diaphragm to produce oscillations in dependence on a transmission signal present at the transducer and PA2 for receiving and converting oscillations thereof into an electrical reception signal,
In this apparatus, a feedback circuit is provided which has an input amplifier, a phase shifter connected in series with the input amplifier and an output amplifier connected in series with the phase shifter. During operation, a reception signal recorded by the reception transducer is applied to the input of the feedback circuit and an output signal of this feedback circuit is applied to the excitation transducer.
This apparatus is a complex oscillatory system composed of a mechanical oscillatory structure, an electromechanical transducer and the feedback circuit. The individual components are not completely isolated electrically and separated mechanically from one another. Both electrical and mechanical coupling occur.
Consequently, the reception signal comprises not only the desired measurement signal, which is ascribed to the oscillation of the respective mechanical oscillatory structure, but also additional signals produced by the aforementioned coupling.
To date, it has not been possible to employ apparatus of this type for measurements in both highly viscous media and in hydrous or viscous foams. When the presettings that were implemented were chosen such that the apparatus operated entirely satisfactorily in one of these applications, problems arose in the other application for example such that reliable excitation of the mechanical oscillatory structure to produce oscillations could not be ensured in one of the applications.
The phase difference generated by the phase shifter corresponds for example to the resonance of the system when the oscillatory structure oscillates in air. However, if the oscillation quality of the system is reduced for any reason, then the consequence of this is that this fixed value of the phase difference no longer exists. There is no frequency at which the mechanical oscillatory structure oscillates having an amplitude different from zero and having a phase difference that is the fixed value. This phase difference cannot be set by the feedback circuit. Incorrect functioning consequently occurs.
A reduction in the oscillation quality occurs, for example, when the movement of the mechanical oscillatory structure is attenuated, for example by the latter being immersed in a viscous medium or in a liquid-containing or viscous foam.
European patent application No. 97107152.7 describes an apparatus for establishing and/or monitoring a predetermined filling level in a container, which apparatus comprises:
The two electrodes of identical form are a first and a second receiver. A further electrode, which serves as a transmitter, is arranged between the first and second receivers. The piezoelectric element has three zones respectively covered by one of the three electrodes. A first zone covered by the first receiver has a polarization direction which runs parallel to a normal to the surface of the piezoelectric element; the other two zones have a polarization direction opposite to the polarization direction of the first zone. In order to excite the apparatus to produce oscillations in the event of resonance thereof, a difference between a signal recorded by the first receiver and a signal recorded by the second receiver is formed, a signal corresponding to the difference is formed and is phase-shifted by a constant, sensor-specific absolute value, and the phase-shifted signal is applied to the transmitter. On account of the different polarization directions of the individual zones, it is a matter of critical importance which signal is subtracted from which. Inadvertent interchanging of the electrical connections of the first and second receivers leads to a non-functional apparatus. Great care must be exercised during the production of such an apparatus. Expensive piezoelectric elements have to be used on account of the zones having different polarization directions. It is necessary to distinguish between the zones of different polarization during the production process and it is necessary to distinguish between the two receivers of identical form and to correctly connect them. This means a high additional outlay of resources and contains sources of error.